This invention relates generally to thermal sensing of low-level radiation of infrared or millimeter wavelengths and more particularly to a pyro-optical pixel structure and focal plane array with means for maintaining a nominal temperature. This invention describes a method of sensing incident radiation using a highly sensitive thermal thin film structure. In its embodiment as an array, a thermal image obtained typically from infrared wavelengths is interrogated using an optical carrier beam and readout with conventional CCD or CMOS silicon imagers.
Work leading to this invention was not funded by the US Government.
Thermal-based sensor systems typically use a pixel that is highly sensitive to temperature differentials. This minute temperature differential is read out using conversion techniques into an electrical signal. The basic components for a thermal imaging system generally include optics for collecting and focusing the incident irradiation from a scene onto an imaging focal plane. A chopper is often included in a thermal imaging system to produce a constant background radiance which provides a reference signal. The electronic processing portion of the thermal imaging system will subtract the reference signal from the total radiance signal to produce an output signal with a minimum background noise level.
The concept of using a pyro-optical material as a sensor to detect radiation by modulating a carrier beam was first disclosed by Elliott in U.S. Pat. No. 4,594,507. This concept is cited as prior art in FIG. 1 as an architectural representation of a system with an optical carrier source 1 and an external radiation source 2 illuminating a pyro-optical pixel 3 with a photodetector 4 to monitor the amplitude of the carrier source 2 modulated by the transmissivity of the pyro-optical pixel 3. In this example the low level radiation source is focused onto the pyro-optical plane 3 through refractory lens 5. The present invention is an improved sensor pixel based on the concept of FIG. 1. The present invention describes a micromachined pixel containing a pyro-optical film integral to a thermally isolated platform and positioned above a temperature-referenced substrate.
The thermal imager of Elliott (U.S. Pat. No. 4,594,507) includes a preferred embodiment of an optically active nematic crystal with a polarizer analyzer that is illuminated from an external light source of unspecified type. This thermal imager operates with an external photodetector of unspecified type illuminated by the external light source through the nematic, temperature-sensitive crystal. The result is an image converter operating with the nematic crystal as the modulator. The system detailed by Elliott operates within an oven typically at 28 deg C. and is specified for imaging infrared irradiation only. Individual pixel structures are not disclosed or claimed and thus items such as separate pixel heaters and micromachined structures are not embodied in this invention. The Elliott system does not use compactness of construction since the external light source and photodetector are not integrated into the structure containing the nematic liquid crystal. Thermal isolation structures surrounding the nematic crystal film and any specific type of optical light source are not mentioned. Performance-enhancing interferometric structures are not mentioned.
Hanson in U.S. Pat. No. 5,512,748 discloses a thermal imaging system containing a focal plane array in which a visible or near-infrared source is used to transfer an image from a transmissivity-modulated pyro-optical film layer onto an associated integrated circuit photodetector. The photodetector integrated into the substrate generates a bias signal representing the total radiance imaged from a remote low-level scene. A thermal sensor is described which contains infrared-sensitive material supported by two bifurcated support arms and nonflexing posts to maintain this film layer above the substrate with a gap therebetween. The thickness of the infrared-sensitive material is not mentioned except to note that it is preferably xe2x80x9cvery thin to enhance it""s response to incident infrared radiation and to allow transmission of electromagnetic energy therethroughxe2x80x9d (col. 7, line 8) without mention of Fabry Perot characteristics. The gap under the sensitive film is said to preferably correspond to xc2xc wavelength of the selected infrared incident radiation wavelength to provide maximum reflection of the infrared from the semiconductor substrate to the infrared-sensitive film. Hanson does not disclose or claim the use of electrical heater elements or any means of temperature control within or without the infrared sensitive pixel. Hanson does not disclose or claim the use of vacuum surrounding the infrared-sensitive pixel.
Owen in U.S. Pat. No. 6,087,661 describes a structure with electrically conducting tetherbeams forming a signal flow path for readout from a pyroelectric pixel material. The tetherbeams further provide a thermal isolation for the pyroelectric sensor microplatform.
Ruffner et al in U.S. Pat. No. 4,751,387 describes the use of a silica foam called aerogel as a solid, thermal isolation film formed between the pyroelectric capacitor and an underlying substrate as part of a specific infrared-sensitive pixel design without claims describing components external to the pixel. The use of pyro-optical sensitive materials is not disclosed or claimed. The Ruffner patent does not mention heating elements or ovens, vacuum conditions, the use of any optical carrier interrogation beams, pyro-optical materials, or the use of performance-enhancing interferometric structures.
Robillard in U.S. Pat. No. 4,751,387 describes an infrared imaging system comprising a pyro-optic film consisting of dichroic liquid crystal coated on a membrane with a means of polarized visible light illumination onto the crystal film. In addition a means for analyzing the polarization of the visible light carrier after reflection from or transmission through the crystal film is included in a system where the readout described is the human eye. Robillard does not disclose or claim any micromachined structures, thermal isolation structures, the use of partial vacuum, ovens, or pixel heaters.
Cross in U.S. Pat. No. 4,994,672 describes an infrared imaging system including a sandwich structure of polarizing pyro-optic material formed over an optically transparent, thermally insulating foam such as silica aerogel. The reflectance (not transmission) of an interrogating light beam is modulated by the temperature of the material and is used to illuminate a pixel image onto a CCD. A container means is provided for enclosing the pyro-optical material and maintaining a stable temperature. The Cross system requires the use of polarized light. The present invention does not utilize the polarization of light. Cross does not modulate the transmission of the interrogating optical beam. Cross does not disclose the use of micromachined pixel structures, performance enhancing interferometric structures, vacuum conditions surrounding the pyro-optic material.
Tuck in U.S. Pat. No. 5,100,218 describes a specific thermal imaging system based on the thermal rotation of polarized light as it is modulated with transmission through a thermally-sensitive liquid crystal. The pyro-optical liquid crystal is separated from the optical source and photodetector by multiple lenses and thus is not a composite, sandwich structure integrating the optical carrier source and photodetectors. Liquid crystal is the only pyro-optical material mentioned. Pyro-optical materials that do not require polarization are not disclosed. Tuck does not disclose any micromachined structures, interferometric structures, any means of controlling ambient temperature, or operation with partial vacuum conditions.
Carr in U.S. Pat. No. 6,091,050 describes a micromachined platform that elevates automatically and without continuing power requirement which is useful for implementing pixels in the present invention. The platform is elevated to a desired level as a result of design and manufacturing controls to create the desired gap between the pyro-optical film and the underlying substrate thereby providing a Fabry Perot interferometric means of enhancing the absorption of incident low-level radiation. Prior state of the art low-level radiation sensors are generally operated using a means of synchronously chopping the incoming low-level radiation to provide an image signal and a reference signal. Electronics for receiving the biased signal and the reference signal and for subtracting the reference signal from the biased signal to obtain an unbiased signal representing radiance differences emitted by objects in the scene is typically implemented in these systems. Carr and Sun in U.S. Pat. No. 5,781,331 describe a micromachined shutter array that can serve as a means of synchronously chopping the incoming low-level radiation with the present invention.
Hanson et al in U.S. Pat. No. 5,486,698 describe an actuation means for periodic thermal coupling of a bolometer or ferroelectric sensing platform to a thermal reference substrate. This actuator operates by electrostatic force which is derived from an external voltage source and eliminates the need for an external mechanical chopper. Hanson does not disclose the use of microactuation as in the present case of a platform with a pyro-optical film.
It is one object of the present invention to provide an improved uncooled, micromachined sensor pixel structure which utilizes a monolithic and self-aligned pixel which responds to thermal radiation. The visible or near infrared transmissivity or reflectivity of the thermally-isolated platform within the pixel is a parameter sensitive to temperature and therefore provides the means of detecting incident, absorbed radiation. The parameters of the pyro-optical film contained within the platform that are temperature sensitive are the optical index of refraction, bandgap absorption, and free carrier absorption. In the schematic system of FIG. 1, the incident low-level radiation 2 typically of wavelength greater than 2 micrometers, when absorbed into the subject pixel, typically heats with a power ranging from femtoWatts to nanoWatts. The high intensity optical beam from source 1 is modulated by the pyro-optical transmissive structure 3 and detected in the readout 4. The resulting thermal modulation index obtained in the readout signal from 4 ranges up to 10 percent. The readout 4 is a photodetector and control circuits for gating and time multiplexing the output signal. For imaging applications, the output electrical signal is a video signal that is processed within 4 to include such functions as synchronous detection. When the optical source 1 is pulsed periodically and then detected synchronously by 4, noise from stray optical sources is eliminated thereby reducing the overall system noise level.
One embodiment of the invention includes a resistive heater element within the thermally-isolated platform integral with the pixel used to maintain a nominal temperature which is modulated by absorbed, incident radiation. The use of a resistive heater element integral with the platform also permits high speed thermal dithering to reduce the effects of thermal response hysteresis.
An advantage of the present pixel is that no polarizing or extinction analyzers are utilized as optical components. An interrogating visible or near infrared light beam is modulated by the platform and signal readout is obtained using an underlying photodetector.
A further uniqueness of the invention is that the resulting thermal modulation index of the signal readout for a given absorbed, incident radiation power is maximized by the use of a first and a second Fabry-Perot film and sandwich structure integral to the pixel illustrated in the FIG. 2 embodiment. Both Fabry Perot structures enhance the response to low-level radiation. The pyro-optical film 31 is itself a first Fabry-Perot structure with an optical thickness to maximize the index of modulation. The thickness optimum for film 31 is less than a wavelength of the carrier beam 21 and is a function of the pyro-optical film dielectric constant and the free carrier absorption at the wavelength of the low-level radiation. The pyro-optical film is typically of high index of refraction and is the primary modulator of optical carrier beam transmissivity. A second Fabry Perot structure is formed by the gap between the micromachined planar platform 31 and the substrate mirror 25 to further increase the index of thermal modulation. The second Fabry Perot structure by creates a node of maximum amplitude for the incident low-level radiation 22 within the platform structure and thus enhances absorption of said low level radiation.
The embodiments of the present invention include an optical focal plane array having thermal sensors formed by a multiplicity of the pixels and with a high degree of reticulation between adjacent pixel platforms to minimize thermal spreading between adjacent pixel elements and to improve the spatial modulation transfer function of the resulting thermal sensors. Tetherbeams are used to support the platform structures above the substrate and with shared support posts to reduce the total array area and to increase the fill factor of pixel utilization. The pixel array processes a low-level image in parallel without the need for line and row scanning circuits within the pixel structures. The image formatting is accomplished by an underlying photodetector array which is typically a CCD or optical CMOS imager.